The need for measuring and recording physiological pressures, for example, in the coronary vessels, has triggered the development of miniaturized devices for enabling the access to the very narrow vessels, such as coronary vessels. Typically a sensor of very small size is mounted on a guide wire, which is inserted in e.g. the femoral artery and guided to the desired point of measurement, e.g. a coronary vessel. There are certain problems associated with the integration of a pressure sensor onto a guide wire suitable for the type of measurements mentioned above. The first and foremost problem is to make the sensor sufficiently small. Also, the number of electrical connections and leads should be minimized, in order to obtain a sufficiently flexible guide wire which can be guided to the desired location through the coronary vessels without too much difficulty.
Sensor and guide wire assemblies in which a sensor is mounted at the distal end of a guide wire are known. In U.S. patent Re. 35,648, which is assigned to the present assignee, an example of such a sensor and guide wire assembly is disclosed, where a sensor guide comprises a sensor element, an electronic unit, a signal transmitting cable connecting the sensor element to the electronic unit, a flexible tube having the cable and the sensor element disposed therein, a solid metal wire (a core wire), and a coil attached to the distal end of the solid wire. The sensor element comprises a pressure sensitive device, e.g. a membrane, with piezoresistive elements connected in a Wheatstone bridge-type of arrangement mounted thereon.
Piezoelectricity refers to the production of electrical charges by the imposition of mechanical stress. The phenomenon is reciprocal. Applying an appropriate electrical field to a piezoelectric material creates a mechanical stress. Piezoelectric acoustic wave sensors apply an oscillating electric field to create a mechanical wave, which propagates through the substrate and is then converted back to an electric field for measurement.
Among the piezoelectic materials that can be used for acoustic wave sensors and devices, the most common are quartz (SiO2), lithium tantalate (LiTaO3), and, to a lesser degree, lithium niobate (LiNbO3). Each has specific advantages and disadvantages, which include cost, temperature dependence, attenuation, and propagation velocity. Other materials with commercial potential include gallium arsenide (GaAs), silicon carbide (SiC), langasite (LGS), zinc oxide (ZnO), aluminum nitride (AlN), lead zirconium titanate (PZT), and polyvinylidene fluoride (PVdF).
The sensors are often made by a photolithographic process. Manufacturing begins by carefully polishing and cleaning the piezoelectric substrate. Metal, usually aluminium, is then deposited uniformly onto the substrate. The device is spin-coated with a photoresist and baked to harden it. It is then exposed to UV light through a mask with opaque areas corresponding to the areas to be metallized on the final device. The exposed areas undergo a chemical change that allows them to be removed with a developing solution. Finally, the remaining photoresist is removed. The pattern of metal remaining on the device is called an interdigital transducer, or IDT. By changing the length, width, position, and thickness of the IDT, the performance of the sensor can be maximized.
If instead the piezoelectric material is in the form of a piezoelectric film conventional thin film technology may be used, starting with a substrate, e.g. a silicon substrate, upon which one or many film(s) and electrode areas are arranged. In addition to the piezoelectric film may be arranged an impedance matching film, an insulating film etc. This will be further discussed in the detailed description.
Acoustic wave devices are described by the mode of wave propagation through or on a piezoelectric substrate. Acoustic waves are distinguished primarily by their velocities and displacement directions; many combinations are possible, depending on the material and boundary conditions. The IDT of each sensor provides the electric field necessary to displace the substrate and thus form an acoustic wave. The wave propagates through the substrate, where it is converted back to an electric field at the IDT on the other side. Transverse, or shear, waves have particle displacements that are normal to the direction of wave propagation and which can be polarized so that the particle displacements are either parallel to or normal to the sensing surface. Shear horizontal wave motion signifies transverse displacements polarized parallel to the sensing surface; shear vertical motion indicates transverse displacements normal to the surface.
A wave propagating through the substrate is called a bulk wave. The most commonly used bulk acoustic wave (BAW) devices are the thickness shear mode (TSM) resonator and the shear-horizontal acoustic plate mode (SH-APM) sensor.
If the wave propagates on the surface of the substrate, it is known as a surface wave. The most widely used surface wave devices are the surface acoustic wave sensor and the shear-horizontal surface acoustic wave (SH-SAW) sensor, also known as the surface transverse wave (STW) sensor.
All acoustic wave devices are sensors in that they are sensitive to perturbations of many different physical parameters. Any change in the characteristics of the path over which the acoustic wave propagates will result in a change in output.
Acoustic wave sensors are utilized in a number of sensing applications, such as, for example, temperature, pressure and/or gas sensing devices and systems e.g. used for measuring tire pressure and temperature for monitoring vehicle tires.
Examples of surface wave sensors include devices such as acoustic wave sensors, which can be utilized to detect the presence of substances, such as chemicals. An acoustic wave device, using for example, surface acoustic waves (SAW) or bulk acoustic waves (BAW), and acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor.
As mentioned above, surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers (IDTs) placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. The selectivity of a surface acoustic wave chemical/biological sensor is generally determined by a selective coating placed on the piezoelectric material. The absorption and/or adsorption of the species to be measured into the selective coating can cause mass loading, elastic, and/or viscoelastic effects on the SAW/BAW device. The change of the acoustic property due to the absorption and/or adsorption of the species can be interpreted as a delay time shift for the delay line surface acoustic wave device or a frequency shift for the resonator (BAW/SAW) acoustic wave device.
An example of an application area for the above sensor may be found in U.S. Pat. No. 6,958,565 that relates to a passive wireless piezoelectric smart tire sensor with reduced size.
Another example of an application area for the above sensor type is found in WO-2005/058166 that relates to a surface or bulk acoustic wave device that can be implanted in a human or animal body to monitor various parameters thereof, e.g. pressure. The device comprises a pair of interdigitated transducers spaced apart over the surface of a piezoelectric substrate that is exposed to the pressure to be monitored. The device is interrogated by a radio-frequency signal being supplied to one of the transducers and detected after reflection by the other transducer. The parameter is measured by comparison of the supplied and received signals.
The object of the present invention is to achieve an improved pressure sensor, in particular for in-vivo measurements, and especially for a sensor and insertion assembly for intravascular measurement of pressure in a living body